1. Field of the Invention
The present invention relates to electrophotographic image forming apparatus using a laser, and, more particularly, to a method of calibrating a laser in such an image forming apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus including a laser, such as a laser printer, uses an optical system to direct the laser beam output from the laser onto a photoconductive (PC) surface such as a PC drum. The optical system typically includes a plurality of separate optical devices such as flat mirrors, lenses and a rotating polygonal mirror. The laser beam output from the laser is projected at a fixed orientation onto the optical system which then redirects and moves the laser beam to cause the laser beam to scan across the PC drum. More particularly, the polygonal mirror is rotated by an associated motor at a known rotational speed. The laser beam output from the laser is directed onto a surface of the polygonal mirror which in turn redirects the laser beam through appropriate lenses and mirrors onto the PC drum. As the polygonal mirror rotates, the angle of projection between the laser beam which impinges thereon and the laser beam which is projected therefrom changes, thus causing the laser beam to scan across the surface of the PC drum. As the laser beam moves from one surface or facet of the polygonal mirror to an adjacent surface or facet of the polygonal mirror, the laser beam moves from an end of a particular scan line to a beginning of a next scan line.
As resolutions and print speeds of laser printers increase, the rotational speed of the polygonal mirror within the printhead must also increase. Since the polygonal mirror and associated motor have a finite mass, the increase in the rotational speed of the polygonal mirror likewise increases the amount of time it takes for the polygonal mirror and associated motor to ramp-up from a zero rotational speed to a known operating rotational speed. Until the polygonal mirror and motor reach the constant operating rotational speed, printing of the print image on the print medium is not carried out. The period of time between actuation of the motor which drives the polygonal mirror and the actual printing of the print image on the print medium, known as the "first copy time", increases as the operating rotational speed of the polygonal mirror increases.
Another factor which also affects the first copy time of the laser printer is the calibration of the laser which is necessary prior to printing the print image on the print medium. The electrical circuitry associated with the laser typically is configured to provide a signal indicative of the absolute power of the laser when the laser is energized.
In a conventional laser printer, the motor driving the polygonal mirror is operated for a predetermined period of time after initial start-up which allows the operating rotational speed of the polygonal mirror to become relatively constant. Thereafter, the laser is energized for a period of time which is sufficient to allow calibration thereof. Printing on the print medium occurs after the laser is calibrated. A problem with this technique is that the laser calibration occurs after the ramp-up period associated with the rotational speed of the polygonal mirror. The "first copy time" therefore increases as a result of the time required for both ramp-up of the polygonal mirror and calibration of the laser.
Another problem is that calibration of the laser typically has been performed by energizing the laser with disregard to the particular location of the laser beam during a scan of the laser beam across the PC drum. The PC drum is discharged at the locations contacted by the laser beam. As a result, the charged toner particles may be attracted to the PC drum and must be subsequently cleaned from the PC drum prior to printing on the print medium. Moreover, some of the toner particles on the PC drum may be attracted to the transfer roller adjacent to the PC drum. If the transfer roller is not cleaned prior to printing of the print image on the print medium, the toner particles on the transfer roller may be transferred to the back of the print medium and thereby contaminate the print medium.
What is needed in the art is a method of reducing the first copy time of the laser printer.